Field of the Invention
The present invention relates to a loader of the type which is used with a crane for lifting and moving heavy weights, and more particularly the present invention relates to a loader which is well suited for lifting blocks in ship construction.
Presently, those blocks assembled in a factory for building ships show a tendency to become heavier and heavier and their weights are now over several hundreds tons. To lift such a heavy block by means of a crane, a large number of eye-pieces are welded first to the block, and the other ends of a large number of wire ropes linked at one ends to these eye-pieces are hung on a hook or hooks of the crane to lift the block. In this lifting operation, it is essential to make the necessary adjustments of the wires so that when the block is lifted the load on each of the wires is the same. In the past, however, such a lifting operation was a very difficult operation and was also attended with danger.
Generally, when lifting blocks by a crane in a shipyard, it has been the practice to attach an eye-piece to each of practically the four corners of blocks of many different shapes and the respective wire ropes fastened to these eye-pieces are brought together at a point on the crane to lift the blocks.
In case of a four-point hanging method in which a block is lifted by means of four wires fastened to the eye-pieces welded to the four corners of the block, at least one fourth of the weight of the block must be supported by the hanger, such as, a shackle attached to the end of each of the wire ropes fastened to the respective eye-pieces, and one fourth of the weight of the block is supported by each wire rope. Consequently, the hanger for supporting one fourth of the weight of a block which is generally a heavy weight material must be made bulky and strong, and the diameter of the wire ropes must be correspondingly large. In addition, it is essential that the eye-pieces are positively attached to a block at the correct positions so that the weight of the block is evenly distributed to the eye-pieces and that the block is made strong enough or reinforced so that the block is not distorted or deformed when it is lifted by means of the eye-pieces.
As a matter of fact, eye-pieces are welded to a block when it is to be lifted and moved from a factory to a dock or when it is to be lifted for assembling work in a dock, and the eye-pieces are removed after the completion of the operation. The material used for the eye-pieces and the labor required for their welding can be considerably reduced by using clamps in place of the eye-pieces. However, for lifting a heavy weight block, e.g., a block which is so heavy that the load acting at each hanging point is greater than 50 tons, a large clamp must be used at each hanging point whose weight is so large that it is difficult to manually operate it. Therefore, to lift heavy blocks which have recently been tending to become heavier and heavier, it is preferable that the number of hanging points is increased to reduce the bearing weight at each hanging point and at the same time measures are taken to eliminate the use of eye-pieces and permit the use of clamps.
In the actual lifting operation of blocks, however, it is not an easy matter to compute the center of gravity of a block and select the position of the clamps so that the weight of the block is equally distributed to a large number of hanging points.
In case of a four-point hanging of a block, the block is lifted slightly as a preliminary operation by means of the wire ropes fastened to the computed hanging points, and the actual lifting of the block is effected only after the skilled operator has confirmed through the preliminary lifting that the load acting on each of the hanging points is the same. The problem is more complicated in the case of mutiple-point lifting of blocks employing more than four hanging points, and there is no insurance that the load of a block can be uniformly distributed to such a large number of hanging points. Therefore, in the presently known arrangements for such multiple-point hanging, a number of balances are arranged in a pyramidal form (a treelike form) in which a pair of balances is suspended from the preceding balance at the ends thereof and in this way the load at each of the multiple hanging points is balanced, or alternately the balances in the tree are replaced with pulleys to balance the load at each of the multiple hanging points.
A disadvantage of these known loaders of the tree configuration is that the loader tends to become long in length and bulky, difficult to operate and more troublesome for maintenance.
The inventors have developed a block loader in which the welding of eye-pieces to a block is eliminated to reduce the labor required for lifting the block and in which a single wire is passed alternately over a large number of movable pulleys arranged in the same plane and a large number of stationary pulleys mounted on the loader proper to accurately distribute the same load to each of the hanging points. In this loader, the movable pulleys are arranged in a row or rows with each row including an odd number of the movable pulleys, and all the movable pulleys are interconnected by a single wire rope passed thereover so that the loader is available for operations ranging from a single-point hanging to a multiple-point hanging equal to the number of the movable pulleys used. In this way, the number of hanging points can be suitably selected in accordance with the shape of a block and at the same time the load can be equally distributed to each of the hanging points without any particular consideration. Thus, this block loader made a great contribution toward solving the operating difficulties encountered in the conventional apparatus.
This block loader is also disadvantageous in that if, for example, more than 30 movable pulleys are used, the frictional resistance between the wire rope and the pulleys is accumulated locally and hence an excessive load acts on a small number of the pulleys.
On the other hand, in a shipyard the individual blocks are usually assembled on a building berth having a certain berth declivity, and therefore it is essential that the block lifted from its horizontal position is brought into an inclined position corresponding to the berth declivity and it is then lowered onto the building berth in this inclined condition. However, with the block loader of the above-described type, the pulleys operate in such a manner that the uniform load always acts on each pulley and thus it is impossible to bring the block into such an inclined position. Further, due to the similar principles, if the center of gravity of the block is not in alignment with the center of the block loader, it is impossible to lift the block into a horizontal position and it is also impossible to adjust the inclination of the block while it is lowered onto the building berth. These deficiencies necessarily mean that the block loader requires the use of necessary auxiliary lifting devices.